Are Olms Blind? How Cave Salamanders Sense Their World

Are olms completely blind?

The olm cannot see images, so in everyday terms it is blind. When an olm hatches, its eyes look fairly normal. Within the first months of development they stop growing and begin to regress, and in adults they lie deep below the skin, where they are rarely visible except in some young animals.

The regressed eyes still contain some light-sensitive cells, however. So does the pineal body, a small light-sensitive organ in the brain found in many amphibians. An adult olm can probably tell light from dark, but it cannot see shapes, movement or color.

There is one notable exception, the black olm of southeastern Slovenia, which is described below.

How do olms sense light without eyes?

Experiments have shown that the olm’s skin itself is sensitive to light, thanks to special light-sensitive pigments in skin cells. When light falls on an olm, it swims away into darkness. This reaction, called negative phototaxis, probably helps keep olms in the safety of the cave and away from the surface, where predators and warm, changeable water await.

Light also changes the olm’s appearance. Although it normally has almost no pigment, an olm kept in the light slowly produces melanin and turns darker, and the skin fades again when the animal returns to darkness.

Smell and taste

Smell and taste are the olm’s most important senses for finding food. Olms can detect very low concentrations of organic substances in the water, and experiments show they judge both the amount and the type of prey by smell better than related amphibians. Their nasal lining is thicker than in surface-dwelling salamanders.

Taste buds sit in the mouth, mostly on the upper side of the tongue, and also near the openings of the gill cavities, where they probably sample the water flowing past. Smell is also thought to play a role in courtship, since males may release chemical signals that attract females.

Hearing and the lateral line

The olm’s inner ear is specialized for picking up sound in water and vibrations from the ground. Behavioral tests suggest it can detect underwater sounds from about 10 Hz to more than 12,000 Hz, with the best sensitivity at around 1,500–2,000 Hz. In a cave without light, hearing may help it locate prey and other sound sources.

Like fish and many aquatic amphibians, the olm also has a lateral line, rows of sense organs on the head and body that detect tiny movements in the surrounding water. This lets it feel small crustaceans swimming nearby, much as a person might feel a draft on their skin.

Electric and magnetic senses

The olm has one of the more unusual sense organs among amphibians. Small ampullary organs on its head act as electroreceptors, detecting weak electric fields. All animals produce faint electric fields when their muscles and nerves work, so this sense may help the olm find prey in total darkness, although its exact use is not fully understood.

Experiments also suggest that olms can sense Earth’s magnetic field. A study published in 2002 found that olms lined up with natural and artificially altered magnetic fields, which may help them orient themselves in large, complicated cave systems.

Why did olms lose their sight?

In permanent darkness, eyes are useless. Building and maintaining eyes, and the brain tissue that processes vision, costs energy, and food in caves is scarce. Biologists think cave animals lose their eyes partly because saving that energy is an advantage, and partly because harmful mutations in eye genes are no longer weeded out by natural selection.

The same thing has happened again and again in caves around the world. Blind, pale cave fish live in Mexico, and North America has several blind cave salamanders, including the Texas blind salamander and the Georgia blind salamander. Charles Darwin mentioned the blind Proteus in On the Origin of Species (1859) as an example of how organs can be reduced in animals living in caves.

Blindness has not held the olm back. It is one of the longest-lived amphibians known; see How long do olms live?. Its strange looks also inspired the legend of the baby dragons of Slovenia.

The black olm: an exception

In 1986, cave researchers found a very different olm in springs near Črnomelj in the Bela krajina region of southeastern Slovenia. The black olm has dark brown to black skin and small but better-developed eyes, as well as a shorter head than the pale olm. It lives in an area of less than 100 km² (39 sq mi), in groundwater close to the surface that can be somewhat warmer than deep cave water.

The black olm is usually treated as a subspecies, Proteus anguinus parkelj. Its pigment and eyes suggest that its ancestors lived nearer to the surface, or that it kept features the cave-dwelling pale olms lost. How much it can actually see is still not clear.